“In this print, Escher uses two-dimensional images to depict objects free of the confines of the three-dimensional world. The image is of a rectangular three-story building. The upper two floors are open at the sides with the top floor and roof supported by pillars. From the viewer’s perspective, all the pillars on the middle floor are the same size at both the front and back, but the pillars at the back are set higher. The viewer also sees by the corners of the top floor that it is at a different angle than the rest of the structure. All these elements make it possible for all the pillars on the middle floor to stand at right angles, yet the pillars at the front support the back side of the top floor while the pillars at the back support the front side. This paradox also allows a ladder to extend from the inside of the middle floor to the outside of the top floor.” (Wikipedia)